The present invention relates to a mechanism for transmitting the movement of a three-dimensional cam in accordance with the operation of an internal combustion engine to a valve lifter.
Generally, a variable valve timing mechanism varies the timing of opening and closing an intake valve or an exhaust valve in an internal combustion engine in accordance with the operation of the internal combustion engine. For example, Unexamined Japanese Patent Publication No. 10-196333 describes a variable valve timing mechanism as shown in FIG. 18. The mechanism includes a three-dimensional cam 402. The height of the cam nose of the cam 402 varies continuously in the axial direction. The cam 402 can be moved axially to alter the operating characteristics of the engine. A valve lifter 404 is provided at the end of a valve (not shown) to follow a cam surface 402a of the cam 402 through a cam follower 406. When the cam 402 moves axially, the location of contact between the cam follower 406 and the cam surface 402a moves axially. As a result, the amount of the valve lift is varied and the valve open-close timing is adjusted.
As shown in FIG. 19, a guide groove 405 is formed on the upper surface 404a of the valve lifter 404 and extends in a direction perpendicular to the axis of the valve lifter 404. A cam follower 406, the cross-section of which is semi-circular, is supported in the guide groove 405. Bearing surfaces 406a of the cam follower 406 contact corresponding supporting surfaces of the guide groove 405. The angle of the cam surface 402 changes as the cam 402 rotates, and the cam follower moves accordingly. This maintains good contact between the cam 402 and the valve lifter 404.
During the rotation of the cam 402, a force urges the cam follower 406 to move in its longitudinal, or axial, direction Z. Therefore, a stopper is provided between the cam follower 406 and the valve lifter 404 to limit the movement of the cam follower 406 in the longitudinal direction Z. That is, an enlarged portion 406c is formed at the longitudinal center of the cam follower 406. A slot 405a is formed in the longitudinal center of the guide groove 405. The enlarged portion 406c is fitted in the slot 405a. The thrust surfaces 406d of the enlarged portion 406c abut against the thrust surfaces 405b of the slot 405a, thus limiting the movement of the cam follower 406 in the longitudinal direction Z.
To achieve smooth valve operation, the surfaces of the cam follower 406 must be ground with high precision. However, the cam follower 406 includes six surfaces that must be machined. As shown in FIGS. 20(A)-(D), the six surfaces include the bearing surfaces 406a, which are separated by the enlarged portion 406c, a cam following surface 406b, the thrust surfaces 406d of the enlarged portion 406c, and a peripheral surface 406e of the enlarged portion 406c. Each of the surfaces must be ground in different steps using a grindstone that is shaped to correspond to each surface. This complicates and increases the grinding work and lowers productivity.
The valve lifter 404 shown in FIG. 21 must be similarly machined. The valve lifter also has surfaces that must each be ground. The surfaces include the support surfaces of the guide groove 405, which are separated by the slot 405a, the thrust surfaces 405b, and the bottom surface 405c of the slot 405a. Five grinding steps are required, which lowers productivity.